This invention relates to a method for heating up the intake air, in particular for an internal combustion engine in the start-up or warm-up-phase.
Heating up the intake air in internal combustion engines, in particular in diesel engines in the start-up and warm-up phases, is necessary for several reasons. At low ambient air temperatures and consequently at low intake air temperatures, an insufficient post-compression temperature and hence an increasing ignition delay occur, in diesel engines in particular, so that the period between the fuel entering the combustion space and igniting becomes too long. In addition, local over-enrichment, incomplete combustion and high pressure gradients occur at low intake temperatures as a result of sudden mixture conversion in the cylinder. The results are greatly increased hydrocarbon emissions in the exhaust gas, knocking of the diesel engine and the disadvantages resulting from this such as severe environmental pollution and increased loading of the powertrain parts.
A heating flange for pre-heating air in an intake line which leads to a diesel internal combustion engine is known from the general prior art according to German patent application DE 100 26 339 A1. It is also known from this document to actuate the heating element in the heating flange by means of a control unit. The controller in this case leaves the current in the heating unit at a constant value and the heating power introduced is determined solely by the switching-on time of the heating element. When the diesel engine is started, the heating flange or the heating element is switched off for a short period in order to make sufficient energy available for the starter from the vehicle electrical system.
A generic method from which the invention proceeds is known from German patent application DE 198 54 077 A1. It is known, from the disclosure provided by this document, to assist the cold start of a diesel engine by means of an air pre-heater, the air being preheated by means of a preglow period and an afterglow period. The preglow period and the afterglow period are interrupted by the start process in which the diesel engine ramps up to starting speed after the starter is activated. During the start process, no energy is supplied to the heating flange. The heating flange is actuated by the engine electronics. Before starting, the preglow period and afterglow period are calculated in the engine electronics from the ambient data. The air temperature and the coolant temperature before the cold start are taken into account here. The preglow period and afterglow period are controlled here only in terms of time, that is to say there is no open-loop or closed-loop control of the current for the operation of the heating element in the intake pipe of the diesel engine.
Today, modern direct injection diesel engines predominantly have electronically controlled injection systems. Cold starting of electronically controlled diesel engines is possible without difficulty down to ambient air temperatures of −15° C. An additional measure, such as heating the intake air by means of a start assist device, is not necessary down to these temperatures. There are however numerous applications where even a modern diesel engine requires a cold start device, such as:                at temperatures below −15° C.,        at temperatures below 0° C. with fuels with a low cetane number,        in applications at altitudes over 1500 m above sea level and below 0° C.,        in applications with base load (hydraulic appliances, fixedly coupled drives) below 0° C.,        in special-purpose vehicles such as mobile cranes, compressors, piste-grooming units, or construction machinery in a temperature range below 0° C.        
The combination of diesel engines with a hydraulic drive in piste-grooming units provides particularly extreme demands on the cold start capabilities of diesel engines. Since manufacturers of these units use increasingly high-powered direct injection diesel engines with a small swept volume which only deploy their full power at full charge air pressure, problems occur when cold starting in regions at relatively high altitude, since no charge air pressure is initially present. If such an engine is started at an altitude of for example 3000 m above sea level and at −15° C., pre-heating devices known today are not sufficient unless the engine is brought up to operating temperature by the coolant temperature by means of a complex auxiliary heating system. For reasons of cost, complete pre-heating, which also includes the drive unit, is only used in arctic applications.
Proceeding from the abovementioned prior art, it is the object of the invention to improve the effect of existing cold start devices by means of improved actuation.
This object is achieved by a method having the claimed features. Further advantageous embodiments of the invention are defined in dependent claims and apparent from the description of the figures.
The invention succeeds primarily by means of variable power adaptation of the heating flange to the prevailing ambient conditions. The preglow phase is divided here into three phases, namely into one phase at full current, one post-heating phase and one start-readiness phase. In the post-heating phase, the heating elements of the heating flange are operated in such a way that the heating flange is kept at a reference temperature. In the post-heating phase, a heat cushion is produced for the start process. After the two first preglow phases, the glow monitoring lamp goes out and, by going out, signals readiness to start. The start-readiness phase then follows. In order to prevent the heating elements from cooling during the start-readiness phase, the heating elements are operated at a further reduced power during the start-readiness phase. If the engine has not been started after a predetermined start-readiness period has expired, the heating elements are switched off.
During the start process, heating of the heating flange is interrupted for a short period but only until the first ignitions assist the running up of the engine to idling speed. As soon as the first ignitions begin, the starter is relieved of load and the vehicle electrical system energy which is available as a result of the relieving of load on the starter is used again to heat the heating elements of the heating flange. By means of the advanced post-heating phase, the temperature of the intake air is prevented from cooling down in the event of the engine turning for a long period.
The following advantages are achieved by means of the invention.
The power-controlled preglow period is only one third of the length of the preglow period of conventional cold start methods.
The afterglow is carried out according to the invention in a power-controlled manner as a function of the coolant temperature and the charge air temperature, the engine speed and the air mass. As a result, at an increased engine speed and an increased air flow rate in the afterglow phase, the electrical heating power can be adapted to the air flow rate so that the charge air temperature does not fall and the true running does not deteriorate even when the engine is cold. The diesel engine can be subjected to load earlier as a result of the adjustment of the heating power during the afterglow period.
It was possible to considerably shorten the interruption of the heating power during the start process. The interruption of the heating power during the start process is now variably adjustable as the interruption is made dependent on the current engine speed. As a result, it is possible to extend the afterglow phase further into the start-up phase as heating begins again when the first ignitions are indicated by an increase in the engine speed. The extent of post-heating is dependent here on the vehicle electrical system voltage. This advanced afterglow period gives better running up assistance to the diesel engine in reaching its idling speed and prevents the charge air temperature from falling in the event of a relatively long turnover period with the starter.
The engine control unit is preferably used to actuate the heating flange. Information about the engine state (engine stationary, engine running, starter on, starter off), the engine speed, the coolant temperature, the charge air temperature, the charge air pressure, the calculated air mass and the vehicle electrical system voltage can constantly be called by means of the engine electronics. By means of correction methods implemented in the engine electronics, the rising or falling charge pressure which has different effects at different altitudes can therefore also be taken into account in the cold start method according to the invention. The electrical power can also be corrected at high altitudes so that the engine can utilize to an optimum degree the low oxygen content which is present there. After the heating bands have initially been supplied with full current during the preglow period and after a reference temperature has been reached, the heating bands of the heating flange are kept at a constant operating temperature by means of reduced heating power. This conserves the battery and protects the heating bands from overloading.
The cold start method according to the invention is suitable for engines having a swept volume of up to 16 liters per heating flange. In corresponding applications of control algorithms in engine electronics, the cold start method according to the invention can also be used for engines which operate with special fuels. These are, for example, engines which operate with kerosene, biodiesel, etc.
The invention will be described in more detail with reference to the drawing figures.